Moriond QCD. Latest Jets Results from the LHC. Klaus Rabbertz, KIT. Proton Structure (PDF) Proton Structure (PDF) Klaus Rabbertz

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1 Latest Jets Results from the LHC Proton Structure (PDF) Proton Structure (PDF), KIT 1

2 Flash Menu Motivation Accelerators and Detectors Jet Algorithms and Calibration Inclusive Jets Dijets and 3-Jets The strong Coupling from Jets Outlook For new results on photons or W/Z bosons plus jets see the dedicated talks this afternoon by R. Lafaye and K. Theofilatos! 2

3 Jets so what? Abundant production of jets hadron colliders are jet laboratories Learn about hard QCD, the proton structure, non-perturbative effects... Proton Structure (PDF) Proton Structure (PDF) Matrix Element Hadrons 3

4 Jets so what? Abundant production of jets hadron colliders are jet laboratories and the strong coupling alpha_s! Proton Structure (PDF) Proton Structure (PDF) Matrix Element Hadrons 4

5 Where are we? Kinematic plane of process scale2 vs. x - Huge new phase space accessible in pp collisions at LHC - Many different final states to examine with high accuracy - A lot of progress on the theory side - Check SM predictions at high scales, but watch out for corrections negligible up to now - Determine the strong coupling and test its running at high scales - Improve on PDFs and precision of SM predictions - Any new features? S. Glazov, Braz.J.Ph. 37 (2007)

6 LHC and Experiments LHC: Collisions of Pb-Pb, p-pb and p-p (23/fb) Ecms = 0.9, 2.36, 2.76, 7, 8 TeV peak inst. lumi almost 8 x 1033 cm-2s-1 ATLAS/CMS global features: Silicon trackers: Calorimetry: Muon chambers: Jet energy scale: Tevatron LHC 2012 Up to η = 2.5 Up to η ~ 5.0 Up to η = % prec. 6

7 Jet Energy Scale Dominant experimental uncertainties for jets! Enormous progress in just three years. Jet Energy Scale (JES) Noise Treatment Pile-Up Treatment ATLAS from 5/fb (2011) (Z+jet channel) Luminosity Jet Energy Resolution (JER)... Approximate development of JEC precision ATLAS, EPJC ; arxiv: ; CONF ; CONF CMS, JME ; JME ; JINST ; DP ; DP D0, arxiv: ; D0 prel

8 Jet Energy Scale and Pile Up μ But: New situation in 2012 at 8 TeV with many pile-up collisions! ATLAS Z μμ candidate with 25 reconstructed primary vertices: (Record beyond 70!) μ CMS from 1.6/fb (8 TeV, 2011) CMS from 5/fb (7 TeV, 2011) Pile-up effect CMS, DP CMS, DP

9 All Inclusive High transverse Momenta Proton Proton Every jet counts 9

10 Inclusive Jets pqcd non-perturbative corrections Agreement with predictions of QCD at NLO over many orders of magnitude up to 2 TeV in jet pt Data at 8 TeV in progress... anti-kt, R=0.6, 7 TeV, 2010 anti-kt, R=0.7, 7 TeV, 2011 ATLAS, PRD 86 (2012) CMS, arxiv: (2012) 10

11 Inclusive Jet Ratios: 0.2 / 0.4 Here: Ratio with different jet sizes R = 0.2 and 0.4 ALICE measurement: following proposal from G. Soyez Emphasizes effects of showering and G. Soyez, PLB698 (2011). hadronization! Cross section, R=0.2 Ratio, R=0.2 / R=0.4 ALICE, arxiv:

12 Inclusive Jet Ratios: 2.76 / 7.0 Here: Ratio at different energies Ecms = 2.76 and 7.0 TeV Result from ATLAS: - cross sections at 2.76 TeV - ratios to 7 TeV - ratio to 7 TeV divided by theory prediction (NLO, CT10, X NP) Shown - study on PDF impact At least partial cancellation of uncertainties more precise comparisons ATLAS, CONF

13 Event Shapes Example: Central Transverse Thrust High pt jets: ATLAS: Advantage Pythia & Alpgen CMS: Advantage Pythia6/8,Herwig++ New: low pt, charged particles: ATLAS: Advantage Pythia6 Z1 CMS, PLB699 (2011). ATLAS, EPJC72 (2012). Better tuning necessary! ATLAS, arxiv:

14 Dijets High Masses Proton Proton 14

15 Dijet Mass Again agreement with predictions of QCD over many orders of magnitude! anti-kt, R=0.6, 7 TeV, 2011 anti-kt, R=0.7, 7 TeV, 2011 CMS, arxiv: (2012) ATLAS, CONF (2012). 15

16 Dijets separated in Rapidity Quantities sensitive to potential deviations from DGLAP evolution at small x Some MC event generators run into problems but also BFKL inspired ones! Large y coverage needed, also useful for WBF tagging jets. Most forward-backward dijet selection All possible dijet pair distances over leading dijet pair distance BFKL inspired DGLAP CMS, EPJC72, ATLAS, JHEP09,

17 Dijet Flavours Study of flavour decomposition of both jets in dijet events Jet flavour determined via template fits to kinematic properties of secondary vertices inside jets BB CC BC Templates differentiate between B, C and light (U) quarks light dijets ~ 80% Well described by MC For most flavour pairs Some discrepancy for B-light (U) contribution at high pt BU CU UU ATLAS, EPJC73,

18 Multijets and αs αs at High Scales Proton Proton 18

19 Inclusive Jet Ratios: 3 / 2 Now: Ratio for different multiplicity Njet = 3 over 2 Avoids direct dependence on PDFs and the RGE of QCD reduces other theor. and exp. uncertainties along the way eliminates luminosity dependence (normalization) M. Wobisch Choices of CMS: Ratio of inclusive 3-jet to 2-jet production Minimal jet pt: Maximal jet rapidity: Average dijet pt as scale Data sample:

20 Comparison to Theory Fits only above 420 GeV to avoid threshold effects αs = NNPDF-NNLO Error bars show full uncertainty including the correlated systematic ones Agreement within uncertainties Similarly described by CT10 and even better by MSTW2008 αs = Discrepancies observed with ABM11 NNPDF-NNLO Not used CMS-PAPER-QCD

21 Determination of αs (NLO) Comparison to other hadron collider experiments CMS-PAPER-QCD Extraction also performed for three subranges in Q Small exp. uncertainty Dominated by th. uncertainty: asymmetric scale uncertainty newly covered range PDF uncertainty exp NNPDF21: αs(mz) = ± CT10: αs(mz) = ± MSTW2008: αs(mz) = ± (ABM11: αs(mz) = ± ) PDF uncertainty: Repeat fit for each NNPDF replica get estimators for μ and σ Scale uncertainty: Repeat fit for six variations of (μr,μf) get maximal deviation 21

22 Outlook Already at 7 TeV LHC opened up new regimes in phase space Results for 8 TeV will come soon Data quality makes jet measurements PRECISION PHYSICS More precise theory required (see next talk) New ideas for analyses are explored Be prepared for interesting comparisons 22

23 Outlook Already at 7 TeV LHC opened up new regimes in phase space Results for 8 TeV will come soon Data quality makes jet measurements PRECISION PHYSICS More precise theory required (see next talk) New ideas for analyses are explored Be prepared for interesting comparisons Thank you for your attention and to the Organizers for the opportunity to speak here! 23

24 Backup Slides 24

25 LHC Aerial View Lake Geneva Airport CMS LHCb ALICE ATLAS 25

26 Jet Algorithms Primary Goal: Establish a good correspondence between: - detector measurements - final state particles and - hard partons Two classes of algorithms: 1. Cone algorithms: Geometrically assign objects to the leading energy flow objects in an event (favorite choice at hadron colliders) 2. Sequential recombination: Repeatedly combine closest pairs of objects (favorite choice at e+e- & ep colliders) Standard at Tevatron: MidPoint Cone Standard at LHC: anti-kt CDF also looked at kt; at LHC also kt, Cam/AC, SISCone in use 26

27 Non-perturbative Corrections Recipe used at Tevatron & LHC: - take LO parton shower (PS) MC - derive corr. for non-pert. (NP) effects, i.e. multiple parton interactions and hadronization assume PS effect small on NLO assume NP effects similar for LO,NLO NP effect: CNPNLO PS effect: CPSNLO Pythia, y < 0.5, R=0.7 POWHEG+Pythia, y < 0.5, R=0.7 NP effect: CNPLO PS effect: CPSLO 20 GeV 1 TeV Observations: - assumptions fine at central rapidity (not shown here) - NP corrections larger for R=0.7 than for y > 2 PS effects visible 20 GeV 1 TeV Figures courtesy of S.Dooling, H.Jung, P.Gunnellini, P.Katsas, A.Knutsson (s. also arxiv: ) 27

28 Data comparison to ABM11 Discrepancies with ABM11 especially below 600 GeV Fits of the strong coupling tend versus the upper edge of the available series in alpha_s With ABM11 NNLO PDF set extrapolation beyond αs = required: Consistent with ABM11 NLO PDF set: Much smaller gluon down to 50% at high x at the same time as preference for smaller couplings To be further discussed/resolved... CMS-PAPER-QCD

29 Comparison of Sensitivity 1 NNPDF21-NNLO ABM11-NNLO CMS-PAPER-QCD

30 Comparison of Sensitivity 2 MSTW2008-NNLO CT10-NNLO CMS-PAPER-QCD

31 CMS R32: Analysis Setup CMS data of 2011 Anti-kT jet algorithm with R = 0.7 Compatible results with R = 0.5 as alternative Selection in rapidity y (1 bin): Ensure tracker coverage Two jets leading in pt must be selected Ensure hard dijet event Minimal transverse momentum pt: Alternative thresholds 50 and 100 GeV checked Alternative relative cut on hardness of 3rd jet tested Minimal average 2-jet <pt1,2> (27 bins): O(2000) 2-jet ev. incl. O(300) 3-jet events above 1 TeV 31

32 CMS R32: Data Treatment Three single-jet triggers (highest pt threshold 370 GeV) Efficiency checked separately for incl. 2- and 3-jet events Particle-flow technique to reconstruct input objects to jet clustering Standard CMS event and jet selection criteria apply (η,pt)-dependent jet energy correction factors, typically: Correction of detector effects using Bayesian iterative unfolding (RooUnfold) Propagation of stat. uncertainties & correlations with MC toy method Cross-checked with SVD unfolding Comparison of directly unfolded ratio R32 versus separate unfolding of inclusive 2- and 3-jet spectra 32

33 CMS R32: Exp. Uncertainties Jet energy correction, known to %: Provided as 16 mutually uncorrelated sources; fully correlated within source; Gaussian behaviour assumed Dominated by absolute scale, followed by high pt extrapolation Unfolding uncertainty accounting for: Variation of jet pt spectral slopes following differences from Pythia6 Z2 (agrees with MadGraph) and Herwig Variation of jet energy resolution (JER) Addition of non-gaussian tails to JER Luminosity (normalization) uncertainty cancels No assumptions on bin-to-bin correlations with respect to y necessary, only 1 bin considered Statistical uncertainties propagated via unfolding 33

34 R_32 Scale Dependence σ_2-jet inclusive σ_3-jet inclusive R_32 fastnlo & G. Flouris 34

35 αs from inclusive Jets (NLO) D0: ± total CDF: D0: M/S: M/S: ± exp. Attention: Evolution of PDFs from low to high Q assumes the validity of the RGEs... Using ATLAS 2010 data CDF, PRL88, 2002 D0, PRD80, 2009 Malaescu/Starovoitov, EPJC72,

36 αs World Summary D0, PLB718 (2012) S. Bethke, 2012: NNLO: ± PDG2012 D0: < ± exp. Tevatron limit, published last year But: Jet data from hadron colliders not included! Jets at NNLO might come this year, see theory talk! A lot of progress by groups around LHC from jets starts here... Th. Gehrmann et al. and N. Glover et al. 36

37 NNLO Scale Dependence Jet pt at NNLO (gluon-gluon only) From talk by N. Glover: Gehrmann- de Ridder, Gehrmann, Glover, Pires 37

38 Process Decomposition 38

39 Achievements 30 years ago... and today! Detector dependent Jets LO Theory Shape not so bad actually! LHC 2010 UA2, PLB 118 (1982). fastnlo 39

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